A wireless communication system, such as LTE ((third generation) Long Term Evolution), LTE-Advanced (Advanced LTE), and WiMAX (Worldwide Interoperability for Microwave Access), employs a cellular configuration in which multiple areas covered by a base station (a transmission station, a transmission device, or eNodeB) are arranged in a cellular manner, thereby enabling an increase in wider communication areas. Additionally, different frequencies are used between neighboring cells (sectors) so that a mobile terminal (a reception station, a mobile station, a mobile terminal device 200, or UE (User Equipment)) can perform communication without being subjected to interferences. However, there has been a problem regarding the frequency use efficiency. For this reason, the same frequency is repeatedly used for each cell (sector) to greatly enhance the frequency use efficiency, necessitating countermeasures against interferences with respect to a mobile terminal present in a cell edge region.
In view of the above situations, a method of reducing or suppressing interferences with respect to a mobile terminal in a cell edge region by performing inter-cell cooperative communication, in which neighboring cells cooperate with each other, has been considered. As such a method, for example, a CoMP (Cooperative Multipoint) transmission method is considered in, for example, Non-Patent Document 1. Additionally, as the CoMP transmission method, joint processing or joint transmission in which the same or different data are cooperatively transmitted between cells, or coordinated scheduling/beamforming in which scheduling or control is cooperatively performed between cells, are considered.
FIGS. 21 and 22 illustrate an example of a mobile terminal moving from a cell central region to a cell edge region. In the case of FIG. 21, a mobile terminal M1 is present in a cell central region served by a base station B1, communicates with the base station B1, and is moving toward a cell edge region between the base station B1 and a base station B2. In the case of FIG. 22, the mobile terminal M1 is present in the cell edge region between the base stations B1 and B2, and is performing cooperative communication with the base stations B1 and B2. It is understood from FIGS. 21 and 22 that the mobile terminal performs communication with a single base station or cooperative communication with multiple base stations depending on the position of the mobile station with respect to the base stations.
On the other hand, more efficient data transmission can be achieved by adaptively controlling, according to a channel state between the base station and the mobile terminal, a modulation scheme, MCS (Modulation and Coding Scheme), the number of spatial multiplexing (layer, rank), precoding weight (precoding matrix), and the like. For example, a method disclosed in Non-Patent Document 2 can be used.
For example, in view of a downlink for data transmission from a base station to a mobile station, to perform such an adaptive control, as shown in FIG. 23, it is necessary for a mobile terminal M to estimate a downlink channel state based on a reference signal RS (a pilot signal or a known signal) which is transmitted from and unique to a base station B and to feed back feedback information FI, such as CSI, to the base station through an uplink for data transmission from the mobile station to the base station.
If a multi-carrier transmission scheme, such as OFDM (Orthogonal Frequency Division Multiplexing) or OFDMA (Orthogonal Frequency Division Multiple Access), is used as a transmission scheme, reference signals scattered over resource elements in frequency and time domains may be used as reference signals unique to a base station, as shown in FIG. 24. As feedback information based on a channel state estimated using such reference signals, a frequency response, CSI (Channel State Information) indicating a channel state, such as SINR (Signal to Interference plus Noise power Ratio), recommended transmission format information with respect to the base station which are CQI (Channel Quality Indicator), RI (Rank Indicator) and/or PMI (Precoding Matrix Index), and the like may be used.